Freestanding, dimensionally stable microporous webs

ABSTRACT

A thin, freestanding, microporous polyolefin web with good heat resistance and dimensional stability includes an inorganic surface layer. A first preferred embodiment is a microporous polyolefin base membrane in which colloidal inorganic particles are present in its bulk structure. Each of second and third preferred embodiments is a thin, freestanding microporous polyolefin web that has an inorganic surface layer containing no organic hydrogen bonding component for the inorganic particles. The inorganic surface layer of the second embodiment is achieved by hydrogen bonding with use of an inorganic acid, and the inorganic surface layer of the third embodiment is achieved by one or both of hydrogen bonding and chemical reaction of the surface groups on the inorganic particles.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/775,604, filed Sep. 11, 2015, which is a U.S. national phaseapplication of International Patent Application No. PCT/US2014/030683,filed Mar. 17, 2014, which claims the benefit of U.S. Patent ApplicationNos. 61/864,448 and 61/801,376, filed August 9 and Mar. 15, 2013,respectively, the contents of all of which are incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates to the formation of freestandingpolyolefin webs that (1) exhibit good in-plane dimensional stability(i.e., low shrinkage) at temperatures both above and below the meltingpoint of the base polymer membrane and (2) maintain shutdown properties.At high temperatures, the pores within the bulk structure of the basepolymer membrane can begin to collapse or shut down and thereby modifyits permeability. Such webs can be used as separators to improve themanufacturability, performance, and safety of energy storage devicessuch as lithium-ion batteries.

BACKGROUND INFORMATION

Separators are an integral part of the performance, safety, and cost oflithium-ion batteries. During normal operation, the principal functionsof the separator are to prevent electronic conduction (i.e., shortcircuit or direct contact) between the anode and cathode whilepermitting ionic conduction by means of the electrolyte. For smallcommercial cells under abuse conditions, such as external short circuitor overcharge, the separator is required to shutdown at temperatureswell below those at which thermal runaway can occur. This requirement isdescribed in Doughty. D, Proceedings of the Advanced Automotive BatteryConference, Honolulu, Hi. (June 2005). Shutdown results from thecollapse of pores in the separator caused by melting and viscous flow ofthe polymer, thus slowing down or stopping ion flow between theelectrodes. Nearly all lithium-ion battery separators containpolyethylene as part of a single- or multi-layer construction so thatshutdown often begins at about 130° C., the melting point ofpolyethylene.

Separators for the lithium-ion market are presently manufactured throughthe use of “dry” or “wet” processes. Celgard LLC and others havedescribed a dry process, in which polypropylene (PP) or polyethylene(PE) is extruded into a thin sheet and subjected to rapid drawdown. Thesheet is then annealed at 10-25° C. below the polymer melting point suchthat crystallite size and orientation are controlled. Next, the sheet israpidly stretched in the machine direction (MD) to achieve slit-likepores or voids. Trilayer PP/PE/PP separators produced by the dry processare commonly used in lithium-ion rechargeable batteries.

Wet process separators composed of polyethylene are produced byextrusion of a plasticizer/polymer mixture at elevated temperature,followed by phase separation, biaxial stretching, and extraction of thepore former (i.e., plasticizer). The resultant separators haveelliptical or spherical pores with good mechanical properties in boththe machine and transverse directions. PE-based separators manufacturedthis way by Toray Tonen Specialty Separator, Asahi Kasel Corp., SKInnovation Co., Ltd., and Entek® Membranes LLC have found wide use inlithium-ion batteries.

More recently, battery failures arising in commercial operation havedemonstrated that shutdown is not a guarantee of safety. The principalreason is that, after shutting down, residual stress and reducedmechanical properties above the polymer melting point can lead toshrinkage, tearing; or pinhole formation. The exposed electrodes canthen touch one another and create an internal short circuit that leadsto more heating, thermal runaway, and explosion.

In the case of large format lithium-ion cells designed for hybrid orplug-in hybrid applications (HEV, PHEV), the benefits of separatorshutdown have been openly questioned because it is difficult toguarantee a sufficient rate and uniformity of shutdown throughout thecomplete cell. This issue is described in Roth, E. P., Proceedings ofLithium Mobile Power Conference, San Diego, Calif. (October 2007). Manycompanies are focused, therefore, on modifying the construction of alithium-ion battery to include (1) a heat-resistant separator or (2) aheat-resistant layer coated on either the electrodes or a conventionalpolyolefin separator. Heat-resistant separators composed of hightemperature polymers (e.g., polyimides, polyester, polyphenylenesulfide) have been produced on a limited basis from solution casting,electrospinning, or other process technologies. In these cases, the highpolymer melting point prevents shutdown at temperatures below 200° C.

U.S. Patent Application Pub. No. US 2012/0145468 describes afreestanding, microporous, ultrahigh molecular weight polyethylene(UHMWPE)-based separator that contains sufficient inorganic fillerparticles to provide low shrinkage while maintaining high porosity attemperatures above the melting point of the polymer matrix (>135° C.).Such freestanding, heat resistant separators have excellent wettabilityand ultralow impedance, but they do not exhibit shutdown propertiesbecause of the high loading level of the inorganic filler.

U.S. Pat. No. 7,638,230 B2 describes a porous heat resistant layercoated onto the negative electrode of a lithium-ion secondary battery.The heat resistant layer is composed of an inorganic filler and apolymer binder. Inorganic fillers include magnesia, titanic, zirconia,or silica, Polymer binders include polyvinylidene fluoride and amodified rubber mixture containing acrylonitrile units. Higher bindercontents negatively impact the high rate discharge characteristics ofthe battery.

U.S. Patent Application Pub. Nos. US 2008/0292968 A1 and US 2009/0111025A1 each describe an organic/inorganic separator in which a poroussubstrate is coated with a mixture of inorganic particles and a polymerbinder to form an active layer on at least one surface of the poroussubstrate. The porous substrate can be a non-woven fabric, a membrane,or a polyolefin-based separator. Inorganic particles are selected from agroup including those that exhibit one or more of dielectric constantgreater than 5, piezoelectricity, and lithium ion conductivity. Selectedpolymer binders are described. The composite separator is said toexhibit excellent thermal safety, dimensional stability, electrochemicalsafety, and lithium ion conductivity, compared to uncoatedpolyolefin-based separators used in lithium-ion batteries. In the caseof certain polymer binders mixed with the inorganic particles, a highdegree of swelling with an electrolyte can result in the surface layer,but rapid wetting or swelling is not achieved in the polyolefinsubstrate.

In the latter two of the above approaches, there is an inorganic-filledlayer that is applied in a secondary coating operation onto the surfaceof an electrode or porous substrate to provide heat resistance andprevent internal short circuits in a battery.

SUMMARY OF THE DISCLOSURE

There has been, heretofore, no consideration of the differentialshrinkage to be expected between the inorganic surface layers and amicroporous polyolefin base membrane as the web is heated, Suchdifferential shrinkage provides stress at the interface between theinorganic surface layer and the polyolefin base membrane such thatfractures or cracks may appear at elevated temperature during shutdownof the separator, No consideration has been given to a coatingformulation that contains colloidal particles that penetrate into thebulk structure of a microporous polyolefin base membrane to improve itsdimensional stability and maintain shutdown properties, whilesimultaneously achieving inorganic surface layers that further preventinternal short circuits and provide good heat resistance and in-planedimensional stability. Moreover, no consideration has been given to aninorganic surface layer containing less than or equal to 5 wt % of anorganic hydrogen bonding component while achieving good in-planedimensional stability above the melting point of the microporouspolyolefin base membrane. The organic hydrogen bonding component can beeither a polymer or a small molecule with multiple acceptor or donorsites for hydrogen bonding. This disclosure also identifiescost-effective methods of application of such coating formulations to amicroporous polyolefin membrane.

A thin, freestanding, microporous polyolefin web with good heatresistance and dimensional stability includes an inorganic surfacelayer. A first preferred embodiment is a microporous polyolefin basemembrane in which colloidal inorganic particles are present in its bulkstructure. This embodiment simultaneously achieves penetration ofcolloidal inorganic particles into the polyolefin base membrane toreduce interfacial stress and maintains shutdown characteristics. Amodification to the first preferred embodiment is a polyolefin web thatincludes an inorganic surface layer containing fumed alumina particlesand less than or equal to 5 wt. % of an organic hydrogen bondingcomponent to achieve a polyolefin web having good in-plane dimensionalstability above the melting point of the polyolefin base membrane. Eachof second and third preferred embodiments is a thin, freestandingmicroporous polyolefin web that has an inorganic surface layercontaining no organic hydrogen bonding component for the inorganicparticles. The inorganic surface layer of the second embodiment isachieved by hydrogen bonding with use of an inorganic acid, and theinorganic surface layer of the third embodiment is achieved by one orboth of hydrogen bonding and chemical reaction of the surface groups onthe inorganic particles.

Applicants believe that mechanical interlocking between the inorganicparticles and the surface pores of the polyolefin base membrane helps tobond the inorganic surface layer to the membrane, thereby creatingreasonable peel strength with limited particle shedding during handlingof the polyolefin web.

Several embodiments of the microporous, freestanding heat resistantpolyolefin web rely upon ultrahigh molecular weight polyethylene(UHMWPE) as a polyolefin base membrane component. The repeat unit ofpolyethylene is (—CH₂CH₂-)_(x), where x represents the average number ofrepeat units in an individual polymer chain. In the case of polyethyleneused in many film and molded part applications, x equals about 10,000;whereas for UHMWPE, x is approximately 150,000. This extreme differencein the number of repeat units is responsible for a higher degree ofchain entanglement and the distinctive properties associated withUHMWPE.

One such property is the ability of UHMWPE to resist material flow underits own weight when heated above its melting point. This phenomenon is aresult of its ultrahigh molecular weight and the associated longrelaxation times, even at elevated temperatures. Although UHMWPE iscommonly available, it is difficult to process into fiber, sheet, ormembrane form. The high melt viscosity requires a compatible plasticizerand a twin screw extruder for disentanglement of the polymer chains suchthat the resultant gel can be processed into a useful form. Thisapproach is commonly referred to as “gel processing.” In many cases,other polyolefins are blended with UHMWPE to lower the molecular weightdistribution to impact properties after extraction of the plasticizer,which extraction results in a porous membrane. The terms “separator” and“web” describe an article that includes an inorganic surface layer and apolyolefin base membrane.

For most of the preferred embodiments described, the microporouspolyolefin membrane is manufactured by combining UHMWPE, high densitypolyethylene (HDPE), and a plasticizer (e.g., mineral oil). A mixture ofUHMWPE and HOPE is blended with the plasticizer in sufficient quantityand extruded to form a homogeneous, cohesive mass. The mass is processedusing blown film, cast film, or calendering methods to give anoil-filled sheet of a reasonable thickness (<250 μm). The oil-filledsheet can be further biaxially oriented to reduce its thickness andeffect its mechanical properties. In an extraction operation, the oil isremoved with a solvent that is subsequently evaporated to produce amicroporous polyolefin membrane that is subsequently coated with aninorganic surface layer.

In a first preferred embodiment, the polyolefin base membrane is passedthrough an aqueous-based dispersion, such as an alcohol/water dispersionof silica. This aqueous-based dispersion contains a number of colloidalparticles that can penetrate through the surface and into the bulkstructure of the polyolefin membrane such that the colloidal particlesare distributed into the polymer matrix. At high temperatures, the poreswithin the bulk structure of the base polyolefin membrane can begin tocollapse or shut down, thereby modifying its permeability, even thoughthere are colloidal inorganic particles distributed throughout the bulkstructure. In addition to penetration of the colloidal particles intothe bulk structure of the polyolefin base membrane, a surface coating ofcontrolled thickness can be formed with wire-wound rods (i.e., Mayerrods, shown in FIG. 1) as the membrane is pulled through theaqueous-based dispersion. The wetted membrane is subsequently dried witha series of air knives and an oven in which hot air is used to evaporatethe solvent. The combination of dip coating and wire-wound rods has,heretofore, not been used to achieve the above-modified polyolefin basemembrane.

In a modification to the first preferred embodiment; the polyolefin basemembrane is passed through an aqueous-based dispersion of (1) fumedalumina or (2) an aqueous-based dispersion of fumed silica combined withcolloidal silica. The alumina or silica particles enable formation of aninorganic surface layer on the polyolefin base membrane through the useof an organic hydrogen bonding component. Preferred organic hydrogenbonding components include both polymers and small molecules withmultiple hydrogen bonding sites. Preferred polymers include polyvinylalcohol (PVOH), polyvinylpyrrolidone (PVP), carboxymethyl cellulose(CMC), polyacrylic acid, and polyethylene oxide. Preferred smallmolecules include catechol, sucrose, tannic acid, maltitol, dimethyloldihydroxyethylene urea (DMDHEU), and pentaerythritol. The resultantseparators exhibit excellent high temperature thermal stability and lowGurley (i.e., high air permeability) values.

In a second preferred embodiment, an inorganic surface layer is formedon the polyolefin base membrane without use of an organic hydrogenbonding component. The polyolefin base membrane is passed through anaqueous-based dispersion of fumed alumina that includes an inorganicacid. The resultant separators exhibit high temperature thermalstability and low Gurley values. Furthermore, the peel strength of theinorganic surface layer can be improved through heat treatment of thecoated polyolefin membrane. Preferred inorganic acids include phosphoricacid (H₃PO₄), pyrophosphoric acid (H₄P₂O₇), metaphosphoric acid (HPO₃),phosphorous acid (HP₃O₃), trimetaphosphoric acid (H₃P₃O₉), boric acid(H₃BO₃), tetraboric acid (H₂B₄O₇), vanadic acid (HVO₃), and molibdicacid (H₂MoO₄).

In a third preferred embodiment, an inorganic surface layer is formed onthe polyolefin base membrane from only inorganic particles, some ofwhich can be colloidal in nature and penetrate into the bulk structure.The polyolefin base membrane is passed through an aqueous-baseddispersion of fumed alumina. The resultant separators exhibit hightemperature thermal stability and low Gurley values. Furthermore, thepeel strength of the inorganic surface layer can be improved throughheat treatment of the coated polyolefin membrane and reaction of thesurface hydroxyl groups on the inorganic particles. Preferred inorganicparticles include metal oxides such as silica, alumina, titania, andzirconia.

Finally, for each of the above embodiments, corona treatment of thepolyolefin-based membrane can improve the overall peel strength of thecoated separator. Applicants believe that oxygen-containing species(e.g., hydroxyl groups) resulting from the corona treatment of thepolyolefin membrane surface hydrogen bond with the inorganic particlesto improve the adhesive strength at the interface between the inorganicsurface layer and the polyolefin membrane.

The resultant microporous, freestanding polyolefin separator asdescribed for each of the three preferred embodiments can be wound orstacked in a package to separate the electrodes in an energy storagedevice, for example, a battery, capacitor, supercapacitor, or fuel cell.Pores can be filled with electrolyte both in the inorganic surfacelayers and throughout the bulk structure of the base polymer membrane.Such separators are beneficial to the manufacture of energy storagedevices, particularly since they combine good heat resistance, in-planedimensional stability, reduced interfacial stress, and shutdowncharacteristics.

Additional aspects and advantages will be apparent from the followingdetailed description of preferred embodiments, which proceeds withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of a Mayer rod used in coating an inorganicsurface layer on a polyolefin membrane in accordance with themethodology disclosed.

FIGS. 2A and 2B present scanning electron micrographs (SEMS) showing theopenings in the surface of a polymer membrane at 20,000× and 40,000×magnifications, respectively.

FIGS. 3 and 4 are cross-sectional schematic diagrams showing,respectively, a polyolefin membrane and a colloid-modified polyolefinmembrane coated with microporous inorganic surface layers.

FIG. 5 presents upper and lower rows of SEM images showing with threedifferent magnifications the surface and bulk structure, respectively,of a polyolefin membrane in which colloidal particles penetrated thesurface of the membrane and into its bulk structure.

FIG. 6 is a diagram of a stainless steel fixture used for electricalresistance measurement of a battery separator.

FIG. 7 is a graph of measured data from which electrical resistance (ER)was determined for an embodiment of a battery separator.

FIG. 8 shows surface and machine direction (MD) fracture SEM images of asilica-coated separator made as described in Example 9.

FIG. 9 shows surface and machine direction (MD) fracture SEM images of acoated separator made as described in Example 16.

FIG. 10 is a bar graph showing the results of peel strength testsperformed on two inorganic surface layer-coated separators subjected tovarious heat treatment conditions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The base membrane used is comprised of a polyolefin matrix. Thepolyolefin most preferably used is an ultrahigh molecular weightpolyethylene (UHMWPE) having an intrinsic viscosity of at least 10deciliter/gram, and preferably in the range from 18-22 deciliters/gram.It is desirable to blend the UHMWPE with other polyolefins such as HDPEor linear low density polyethylene (LLDPE) to impact the shutdownproperties of the membrane. Membranes can also be manufactured fromother polyolefins or their blends, such as, for example,ethylene-propylene copolymers, polypropylene, and polymethyl pentene.

The plasticizer employed is a nonevaporative solvent for the polymer andis preferably a liquid at room temperature. The plasticizer has littleor no solvating effect on the polymer at room temperature; it performsits solvating action at temperatures at or above the softeningtemperature of the polymer. For UHMWPE, the solvating temperature wouldbe above about 160° C., and preferably in the range of between about189° C. and about 240° C. It is preferred to use a processing oil, suchas a paraffinic oil, naphthenic oil, aromatic oil, or a mixture of twoor more such oils. Examples of suitable processing oils include: oilssold by Shell Oil Company, such as Gravex™ 942; oils sold by CalumetLubricants, such as Hydracal™ 800; and oils sold by Nynas Inc., such asHR Tufflo® 750.

The polymer/oil mixture is extruded through a sheet die or annular die,and then it is biaxially oriented to form a thin, oil-filled sheet. Anysolvent that is compatible with the oil can be used for the extractionstep, provided it has a boiling point that makes it practical toseparate the solvent from the plasticizer by distillation. Such solventsinclude 1,1,2 trichloroethylene; perchloroethylene; 1,2-dichloroethane;1,1,1-trichloroethane; 1,1,2-trichloroethane; methylene chloride;chloroform; 1,1,2-trichloro-1,2,2-trifluoroethane; isopropyl alcohol;diethyl ether; acetone; hexane; heptane; and toluene. In some cases, itis desirable to select the processing oil such that any residual oil inthe polyolefin membrane after extraction is electrochemically inactive.

FIGS. 2A and 2B show scanning electron micrographs (SEMS) of anembodiment of a 16 μm polymer membrane at 20,000× and 40.000×magnification, respectively. FIGS. 2A and 2B show that the openings inthe surface of the polymer membrane are typically less than 250 nm indiameter, though many are smaller.

The coating formulation used in the first preferred embodiment iscomposed of inorganic particles dispersed in an aqueous-based dispersionin which greater than 50% water is counted in the liquid phase. Theinorganic particles are typically charge stabilized and stay suspendedin the alcohol/water mixture. An organic hydrogen bonding component,such as low molecular weight, water-soluble polymer, is also present. Itis desirable to choose a polymer with numerous hydrogen bonding sites tominimize its concentration, yet achieve a robust, microporous inorganicsurface layer that does not easily shed inorganic particles. Polyvinylalcohol is a preferred organic hydrogen bonding component such thatfewer than 5 parts of PVOH can be used with 95 parts or more of theinorganic particles. This organic hydrogen bonding component impartshigh peel strength and good in-plane dimensional stability to the coatedmembrane, while being suitable for coating application from anaqueous-based dispersion.

FIG. 3 shows a cross-sectional schematic of a polyolefin membrane coatedwith microporous inorganic surface layers. FIG. 4 shows thecross-sectional schematic of FIG. 3 but with a colloid-modifiedpolyolefin membrane.

In addition to controlling the amount of organic hydrogen bondingcomponent and inorganic particles in the coating formulation, applicantsbelieve it is important to control the particle size distribution of theinorganic particles. Furthermore, the coating formulation was carefullyapplied to the polyolefin base membrane to control the thickness of theresultant inorganic surface layer.

Examples 1 and 2 demonstrate that the colloidal particles penetratethrough the surface and into the bulk structure of the polyolefinmembrane.

Example 1

A 16 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® KLP (Entek Membranes LLC,Oregon) (see FIGS. 2A and 2B) was used as a polyolefin membrane forcoating. The 16 μm Entek® KLP membrane, before coating, is referred toherein as the control. The membrane was dipped through a 275 partisopropyl alcohol:1000 part water solution containing colloidal silica(LUDOX; Sigma-Aldrich Co. LLC) at the following concentrations: 5, 10,and 20 wt. %. Two #00 Mayer rods were used (one on each surface of themembrane) to remove the wet surface layer, and the membrane was thendried with a series of air knives and transported through a verticaloven set at 120° C.

The samples were examined by scanning electron microscopy and energydispersive x-ray analysis to show that colloidal silica particlespenetrated the membrane surface and were present in the bulk structure,as shown with three different magnifications in the SEMs arranged in thebottom row (MD fracture) of FIG. 5.

Example 2

The thermal shrinkage values of the colloidal-modified separators inExample 1 were compared with the 16 μm Entek® KLP control. Three 100mm×100 mm samples were cut from each separator type. The sample groupswere held together with a small binder clip fixed in a corner. Thesamples were then suspended in an oven at 200° C. for 30 minutes. Afterclosure of the oven, it was evacuated and then backfilled with argon forthis test. Upon removal, the samples were cooled to room temperature andthen measured to determine their shrinkage in the machine direction (MD)and the transverse direction (TD). The results in Table 1 show thatthere was a substantial reduction in transverse direction shrinkage asthe separators were exposed to higher concentrations of colloidalsilica.

TABLE 1 200° C. shrinkage results 200° C. shrinkage Sample MD % TD % 16μm Entek KLP control 76.7 69.2 5% Ludox 71.1 58.7 10% Ludox 68.0 57.820% Ludox 65.6 49.5

Examples 3-17 relate to inorganic surface layer coating formulations inaccordance with a first preferred embodiment.

Example 3

A 16 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® KLP (Entek Membranes LLC,Oregon) was coated with an aqueous-based dispersion that contained thefollowing:

 25 g Polyvinyl alcohol (87-89% hydrolyzed; MW = 13-23K; Aldrich) 610 gDistilled water 275 g Isopropanol  59 g LUDOX HS-4G (40 wt. % colloidalsilica; Sigma-Aldrich Co. LLC) 1484 g  CAB-O-SPERSE 1030 K (30 wt. %fumed silica; Cabot Corporation).

The coating dispersion contained 20% solids with a 90:5:51030K:LUDOX:PVOH mass ratio. The CAB-O-SPERSE 1030K is an aqueousdispersion of fumed silica with a mean aggregate size of 150 nm and asurface area of 90 m²/g. Two #7 Mayer rods were used (one on eachsurface of the membrane) in the dip coating operation (residence timeabout 7 seconds), and the wetted separator was dried as described inExample 1. Shrinkage values of the coated separator in the machinedirection (MD) and the transverse direction (TD) were determined, asdescribed in Example 2. The separator had a final thickness of 20.2 μm,a basis weight increase of 3.9 g/m², a thermal shrinkage of 3.1% in theMD and 2.7% in the TD, and a Gurley value of 483 seconds. A Gurley valueis a measure of air permeability determined with use of a Gurley®densometer Model 4340, which measures the time in seconds (s) for 100 ccof air to pass through a 6.45 cm² membrane at an applied pressure of1215 Pa.

Example 4

Separator electrical resistance (ER) was measured in a glove box using afixture with stainless steel electrodes, lithium-ion electrolyte (1MLiPF₆ in 1:1 Ethylene Carbonate:Ethyl Methyl Carbonate (EMC)), and animpedance analyzer (Carry PC4 750) operating over a frequency range of100 kHz to 1 kHz. FIG. 6 is a diagram of the stainless steel fixtureused for electrical resistance measurement. The real component of themeasured impedance at 100 kHz was plotted for 1, 2, and 3 layers ofseparator. FIG. 7 is a graph of the measured data from which electricalresistance (ER) was determined. The slope of the linear fit of measuredresistance to the number of separator layers was used as the electricalresistance of the separator.

The areal resistance, electrical resistivity, and MacMullin Numbermeasurements were made for the separator samples described in Examples 1and 3. A comparison to the 16 μm Entek® KLP base membrane is shown inTable 2.

TABLE 2 Electrical resistivity, areal resistance, and MacMullin numberdata Average Areal Test material description: Thick- Resis- Resis-MacMullin Coated Entek Series (Test ness tance tivity Numberelectrolyte: 1.0M LiPF6, Units 1:1 EC:EMC) mm Ω-cm² Ω-cm dim'lessJ161X831, 16 μm KLP, 0.0181 3.13 1730 13.0 base membrane J161X833, 16 μmKLP, 0.0181 2.85 1572 11.8 base membrane CDL130225.001, 0.0178 3.16 177613.4 5% LUDOX CDL130226.004, 0.0189 3.88 2051 15.4 10% LUDOXCDL130204.001, 0.0197 5.61 2848 21.4 20% LUDOX CDL130227.006, 90/5/50.0208 3.31 1595 12.0 1030K/LUDOX/PVOH

Example 5

A 16 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® CLP (Entek Membranes LLC,Oregon) was coated with an aqueous-based dispersion that contained thefollowing:

 14.5 g Polyvinyl alcohol (PVOH, 87-89% hydrolyzed; MW = 13-23K;Aldrich) 1000 g Distilled water  275 g Isopropanol 1172 g CAB-O-SPERSEPG 008 (40 wt. % alumina; Cabot Corporation).

The coating dispersion contained 19.6 wt. % solids with a 97/3alumina/polyvinyl alcohol (PVOH) mass ratio. The CAB-O-SPERSE PG 008 isan aqueous dispersion of fumed alumina with a mean aggregate size of 130nm and a surface area of 81 m²/g.

The separator was dip-coated through a bath containing the aqueous-baseddispersion, and the thickness of the wet layer was controlled on eachside with a #14 Mayer rod. The wetted separator was then dried with aseries of air knives and transported through a vertical oven set at 80°C. and wound on a plastic core, prior to testing. The separator had afinal thickness of 20.0 μm and a Gurley value of 464 seconds. The basisweight increased 5 g/m² after the coating and drying operations.

The thermal shrinkage of the coated separator was determined. Three 100mm×100 mm samples were cut from the separator. The samples were thensuspended in an oven at 200° C. for 30 minutes. After closure of theoven, it was evacuated and then backfilled with argon for this test.Upon removal, the samples were cooled to room temperature and thenmeasured to determine their shrinkage in the machine direction (MD) andthe transverse direction (TD). Results showed average shrinkage valuesof 3.4% in the MD and 2.2% in the TD.

Example 6

A 12 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® CLP (Entek Membranes LLC,Oregon) was coated with an aqueous-based dispersion that contained thefollowing:

 7.14 g Polyvinyl alcohol (PVOH; 87-89% hydrolyzed; MW = 13-23K;Aldrich) 1000 g Distilled water  275 g Isopropanol 1172 g CAB-O-SPERSEPG 008 (40 wt. % alumina; Cabot Corporation).

The coating dispersion contained 19.4 wt. % solids with a 98.5/1.5alumina/polyvinyl alcohol (PVOH) mass ratio. After dip coating theseparator through a bath containing the aqueous-based dispersion, twoMayer rods (#9, #12, or #14) were used to control the wet layerthickness on each side. The wetted separator was then dried with aseries of air knives and transported through a vertical oven set at 80°C. and wound on a plastic core, prior to testing.

Cut samples were then suspended in an oven at 200° C. for 30 minutes.Upon cooling, sample shrinkage in the machine direction (MD) and thetransverse direction (TD) was determined, as described in Example 5.Table 3 shows the separator coating pickup, high temperature thermalstability, and Gurley values for the coated separators using variousMayer rods. The results illustrate that the coating thickness could becontrolled while maintaining excellent high temperature thermalstability and low Gurley values. Additionally, increasing the thicknessof the inorganic surface layer did not negatively affect the Gurleyvalues of the separators using this formulation.

TABLE 3 Coated separator characteristics Wt. Thickness pickup Pickup200° C. shrinkage Gurley Composition Rod # (g/m²) (μm) MD % TD %(sec/100 ml) 98.5/1.5 Alumina/PVOH #09/09 4.72 3.0 2.9 2.9 410 98.5/1.5Alumina/PVOH #12/12 7.56 5.4 2.2 2.2 389 98.5/1.5 Alumina/PVOH #14/148.33 6.0 2.7 2.7 378

Example 7

A 16 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® KLP (Entek Membranes LLC,Oregon) was coated with an aqueous-based dispersion containing thefollowing:

 62 g Polyvinylpyrrolidone (LUVETEC K115, 10% solution in water: MW =2.2 million; BASF) 1242 g  Distilled water 258 g Isopropanol 500 gCAB-O-SPERSE PG 008 (40 wt. % alumina; Cabot Corporation).

The coating dispersion contained 10 wt. % solids with a 97/3alumina/polyvinylpyrrolidone (PVP) mass ratio. Two #14 Mayer rods wereused (one on each surface of the membrane) to control the wet layerthickness; and the separator was then dried with a series of air knives,transported through a vertical oven set at 80° C., and wound on aplastic core, prior to testing. Shrinkage values of the coated separatorin the machine direction (MD) and the transverse direction (TD) weredetermined, as described in Example 5. Table 4 shows the separatorcoating pickup, high temperature thermal stability, and Gurley valuesfor the coated separator. Results showed excellent high temperaturethermal stability and low Gurley values can be obtained for separatorswith inorganic surface layers containing PVP as the organic hydrogenbonding component.

TABLE 4 Coated separator characteristics Wt. Thickness pickup Pickup200° C. shrinkage Gurley Composition Rod # (g/m²) (μm) MD % TD %(sec/100 ml) 97/3 Alumina/PVP #14/14 4.66 3.6 3.9 3.4 402

Example 8

A 16 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® KLP (Entek Membranes LLC,Oregon) was coated with an aqueous-based dispersion that contained thefollowing:

 62 g Polyvinylpyrrolidone (LUVETEC K115, 10% solution in water; MW =2.2 million; BASF) 942 g Distilled water 258 g Isopropanol 800 gAERODISP W 925 (25 wt. % alumina; Evonik Corporation).

The coating dispersion contained 10 wt. % solids with a 97/3alumina/polyvinylpyrrolidone (PVP) mass ratio. The AERODISP W 925 is anaqueous dispersion of fumed alumina with a mean aggregate size of 100 nmand a surface area of 81 m²/g. Two #14 Mayer rods were used (one on eachsurface of the membrane) in the dip coating operation, and the coatedseparator was dried as described in Example 5. Shrinkage values at hightemperatures were determined by suspending in an oven at 200° C. for 30minutes and then measuring the change in machine and transversedimensions upon cooling (see Example 5). Characteristics of the coatedseparator are described in Table 5.

TABLE 5 Coated separator characteristics Wt. Thickness pickup Pickup200° C. shrinkage Gurley Composition Rod # (g/m²) (μm) MD % TD %(sec/100 ml) 97/3 Alumina/PVP #14/14 4.14 4.8 5.5 3.9 362

Example 9

A 12 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® GLP (Entek Membranes LLC,Oregon) was coated with an aqueous-based dispersion containing:

 60 g Polyvinylpyrrolidone (LUVETEC K115, 10% solution in water; MW =2.2 million; BASF) 1058 g  Distilled water 247 g Isopropanol  35 gColloidal silica (LUDOX; Sigma-Aldrich Co. LLC) 600 g Fumed silicadispersion (CAB-O-SPERSE 1030 K; 30 wt. % solids; Cabot Corporation).

The coating dispersion contained 10 wt. % solids with a 90/7/3 fumedsilica/colloidal silica/polyvinylpyrrolidone (PVP) mass ratio. Two Mayerrods were used (one on each surface of the membrane) to control the wetlayer thickness, and dried as described in Example 5. The separator hada final thickness of 19.8 μm, a weight pickup of 2.9 g/m², and a Gurleyvalue of 560 seconds. Surface and MD fracture SEM images of the coatingsare shown in FIG. 8.

Example 10

A 12 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® GLP (Entek Membranes LLC,Oregon) was coated with aqueous-based dispersions containing thefollowing:

 14.5 g Polyvinyl alcohol (Kuraray; Mowil 4-88, 88% hydrolyzed) 1275 gDistilled water 1172 g Cabosperse PG008 (40 wt. % alumina; CabotCorporation),with varying concentrations of surfactant (Dow Q2-5211; 0 wt. %, 0.01wt,%, 0.1 wt. %, and 0.2 wt. %).

The coating dispersion contained 19.6 wt. % solids with a 97/3alumina/polyvinyl alcohol mass ratio. Two #09 Mayer rods were used (oneon each surface of the membrane) to control the wet layer thickness; andthe separator was then dried with a series of air knives, transportedthrough a vertical oven set at 80° C., and wound on a plastic core,prior to testing. Shrinkage of the coated separator in the machinedirection (MD) and the transverse direction (TD) was determined, asdescribed in Example 5. Table 8 presents the coating pickup, hightemperature thermal stability, and Gurley values for the coatedseparator. The data show that the inorganic surface layer exhibitsexcellent high temperature thermal stability, irrespective of whether asurfactant or isopropanol is present in the coating formulation.

TABLE 8 Coated separator characteristics Wt. Thickness pickup pickup200° C. shrinkage Gurley Composition Rod # (g/m²) (μm) MD % TD %(sec/100 ml) 97/3 Alumina/PVOH, no IPA, #09/09 5.2 3.8 2.4 2.1 315 nosurfactant 97/3 Alumina/PVOH, 0.02% #09/09 5.5 3.7 2.6 1.9 344surfactant 97/3 Alumina/PVOH, 0.1% #09/09 5.6 3.6 2.6 2.6 393 surfactant97/3 Alumina/PVOH, 0.2% #09/09 6.3 4.3 2.9 2.7 359 surfactant

Example 11

A 12 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® GLP (Entek Membranes LLC,Oregon) was coated with aqueous-based dispersions containing thefollowing:

 14.5 g Polyvinyl alcohol (Kuraray; Mowil 4-88, 88% hydrolyzed) 1172 gCabosperse PG008 (40 wt. % alumina; Cabot Corporation).

The coating dispersion contained 40.7 wt. % solids with a 97/3alumina/polyvinyl alcohol mass ratio. Two Mayer rods were used (one oneach surface of the membrane) to control the wet layer thickness; andthe separator was then dried with a series of air knives, transportedthrough a vertical oven set at 80° C., and wound on a plastic core,prior to testing. Shrinkage of the coated separator in the machinedirection (MD) and the transverse direction (TD) was determined, asdescribed in Example 5. Table 9 shows the coating pickup, hightemperature thermal stability, and Gurley values for the coatedseparator. This example illustrates that PVOH can be directly dissolvedinto the aqueous-based dispersion to obtain an inorganic surface layerwith high temperature thermal stability,

TABLE 9 Coated separator characteristics Wt. Thickness pickup pickup200° C. shrinkage Gurley Composition Rod # (g/m²) (μm) MD % TD %(sec/100 ml) 97/3 Alumina/PVOH #07/07 8.04 5.3 2.4 2.1 370 97/3Alumina/PVOH #09/09 13.84 10.6 2.4 1.4 379

Example 12

A 32 μm thick microporous ultrahigh molecular weightpolyethylene-containing separator composed of two individual 16 μm thickmembrane layers, Entek® HPIP (Entek Membranes LLC, Oregon) was coatedwith an aqueous-based dispersion containing the following:

 14.5 g Polyvinyl alcohol (Kuraray; Mowil 4-88, 88% hydrolyzed) 1000 gDistilled water  275 g Isopropanol 1172 g Cabosperse PG008 (40 wt. %alumina; Cabot Corporation).

The coating dispersion contained 20 wt. % solids with a 97/3alumina/polyvinyl alcohol mass ratio. Two Mayer rods were used (one oneach side of the membrane) to control the wet layer thickness; and theseparator was then dried with a series of air knives and transportedthrough a vertical oven set at 80° C., as described in Example 5. Theseparator was then split into its individual layers, leaving one sideuncoated and one side coated for each layer. Each layer was wound onto aplastic core prior to testing. Table 10 shows the coating pickup andGurley values for the coated separators. This example illustrates anextremely efficient method of manufacturing a separator with aninorganic surface layer on only one side of the polyolefin membrane.

TABLE 10 Coated separator characteristics Wt. Thickness pickup pickupGurley Composition Rod # (g/m²) (μm) (sec/100 ml) 97/3 Alumina/ #14 4.43.2 209 PVOH, Side 1 97/3 Alumina/ #14 4.3 3.5 216 PVOH, Side 2

Example 13

A 16 μm thick, microporous polyethylene-based separator prepared using adry process (Foresight Separator, Foresight Energy Technologies Co. Ltd)was coated with an aqueous-based dispersion containing the following:

100 g Selvol 21-205 Polyvinyl alcohol aqueous solution (21 wt. %; 88%hydrolyzed; Sekisui) 205 g Distilled water 1697.5 g   Cabosperse PG008(40 wt. % alumina; Cabot Corporation).

The coating dispersion contained 35 wt. % solids with a 97/3alumina/PVOH mass ratio. After dip coating the separator into a bathcontaining the alumina dispersion; two Mayer rods (#5, #7, or #10) wereused (one on each surface of the membrane) to control the wet layerthickness, and the separator was then dried with a series of air knives,transported through a vertical oven set at 80° C., and wound on aplastic core, prior to testing, Shrinkage of the coated separator in themachine direction (MD) and the transverse direction (TO) was determined,as described in Example 5. Table 11 shows the coating pickup, hightemperature thermal stability, and Gurley values for the coatedseparator.

TABLE 11 Coated separator characteristics Wt. Thickness pickup pickup200° C. shrinkage Gurley Composition Rod # (g/m²) (μm) MD % TD %(sec/100 ml) 97/3 alumina/PVOH #05/05 2.24 2.2 8.2 1.1 267 97/3alumina/PVOH #07/07 4.55 4.2 7.2 1.1 340 97/3 alumina/PVOH #10/10 9.678.9 2.7 1.6 347

Example 14

A 16 μm thick, microporous ultrahigh molecular weight polyethylene-basedseparator, Entek® KLP (Entek Membranes LLC, Oregon) was coated with anaqueous-based dispersion containing the following:

7.1 g  Selvol 21-205 Polyvinyl alcohol aqueous solution (21 wt. %; 88%hydrolyzed Sekisui) 60 g Distilled water 70 g Boehmite (AlO—OH) [5 wt. %in water; see J. Appl. Chem. Biotechnol. 1973, 23, 803-09 forpreparation] 112.5 g   Cabosperse PG008 (40 wt. % alumina; CabotCorporation).

The coating dispersion contained 20 wt. % solids with a 90/7/3alumina/boehmite/PVOH mass ratio. The separator was dip-coated into abath containing the aqueous-based dispersion. The coated polyolefinmembrane was then dried in an oven set to 80° C. for 30 minutes prior totesting. Shrinkage of the coated separator in the machine direction (MD)and the transverse direction (TO) was determined, as described inExample 5. Table 12 shows the coating pickup, high temperature thermalstability, and Gurley values for the coated separator.

TABLE 12 Coated separator characteristics Wt. Thickness Coating pickuppickup 200° C. shrinkage Gurley Composition (g/m²) (μm) MD % TD %(sec/100 ml) 90/7/3 alumina/ 11.5 8.6 2.8 1.6 345 boehmite/PVOH

Example 15

A 12 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® GLP (Entek Membranes LLC,Oregon) was coated with an aqueous-based dispersion that contained thefollowing:

 53.4 g Pentaerythritol (Aldrich) 1116 g Distilled water  295 gIsopropanol 1200 g CAB-O-SPERSE PG 008 (40 wt. % alumina; CabotCorporation).

The coating dispersion contained 20 wt. % solids with a 90/10alumina/pentaerythritol mass ratio. Two #09 Mayer rods (one on eachsurface of the membrane) were used to control the wet layer thickness;and the separator was then dried with a series of air knives,transported through a vertical oven set at 80° C., and wound on aplastic core, prior to testing. Shrinkage values of the coated separatorin the machine direction (MD) and the transverse direction (TD) weredetermined, as described in Example 5. Table 13 shows the separatorcoating pickup, high temperature thermal stability, and Gurley valuesfor the coated separator.

TABLE 13 Coated separator characteristics Wt. Thickness pickup pickup200° C. shrinkage Gurley Composition Rod # (g/m²) (μm) MD % TD %(sec/100 ml) 90/10 Alumina/Pentaerythritol #09/09 5.67 3.0 4.5 3.1 406

Example 16

A 12 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® GLS' (Entek Membranes LLC,Oregon) was coated with aqueous-based dispersions that contained 20 wt %solids with 90/10 and 80/20 alumina/sucrose mass ratios. Compositions ofeach of the aqueous-based dispersions prepared are described in Table14.

TABLE 14 Dispersion Compositions Isopropyl PG008 Description of Sucrose(g) DI water Alcohol Dispersion (g) coating (Aldrich) (g) (g) Cabot)90/10 mass ratio 20 420 110 450 Alumina/Sucrose 80/20 mass ratio 40 450110 400 Alumina/Sucrose

Two #09 Mayer rods were used (one on each surface of the membrane) tocontrol the wet layer thickness; and the separator was then dried with aseries of air knives, transported through a vertical oven set at 80° C.,and wound on a plastic core, prior to testing. Shrinkage of the coatedseparator in the machine direction (MD) and the transverse direction(TD) was determined, as described in Example 5. Table 15 shows thecoating pickup, high temperature thermal stability, and Gurley valuesfor the coated separator. Surface and MD fracture SEM images of a 90/10alumina/sucrose coating mass ratio is shown in FIG. 9. This exampleillustrates that small molecules with high hydrogen bonding abilitiescan be incorporated into the inorganic surface layer to yield hightemperature thermal stability of the coated separator.

TABLE 15 Coated separator characteristics Wt. pickup 200° C. shrinkageGurley Composition Rod # (g/m²) MD % TD % (sec/100 ml) 90/10 alumina/#09/09 5.5 2.4 1.2 401 sucrose 80/20 alumina/ #09/09 5.0 23.6 26.2 437sucrose

Example 17

A 12 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® GLP (Entek Membranes LLC,Oregon) was coated with aqueous-based dispersions that contained 20 wt.% solids with 90/10, 80/20, and 70/30 alumina/maltitol mass ratios.Compositions of each of the coating dispersions prepared are describedin Table 16.

TABLE 16 Dispersion Compositions Isopropyl PG008 Description of Maltitol(g) DI water Alcohol Dispersion (g) coating (Aldrich) (g) (g) Cabot)90/10 mass ratio 20 420 110 450 Alumina/Maltitol 80/20 mass ratio 40 450110 400 Alumina/Maltitol 70/30 mass ratio 60 480 110 350Alumina/Maltitol

Two #09 Mayer rods were used (one on each surface of the membrane) tocontrol the wet layer thickness; and the separator was then dried with aseries of air knives, transported through a vertical oven set at 80° C.,and wound on a plastic core, prior to testing. Shrinkage of the coatedseparator in the machine direction (MD) and the transverse direction(TD) was determined, as described in Example 5. Table 17 shows thecoating pickup, high temperature thermal stability, and Gurley valuesfor the coated separator. This example further shows that smallmolecules with high hydrogen bonding abilities can be incorporated intothe inorganic surface layer to yield high temperature thermal stabilityof the coated separator.

TABLE 17 Coated separator characteristics Wt. pickup 200° C. shrinkageGurley Composition Rod # (g/m²) MD % TD % (sec/100 ml) 90/10 alumina/#09/09 5.08 2.6 3.1 389 maltitol 80/20 alumina/ #09/09 5.63 8.8 22.9 393maltitol 70/30 alumina/ #09/09 4.88 27.2 39.3 644 maltitol

Example 18 related to an inorganic surface layer coating formulationachieved by hydrogen bonding with use of an inorganic acid, inaccordance with a second preferred embodiment.

Example 18

A 16 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® KLP (Entek Membranes LLC,Oregon) was coated with aqueous-based dispersions containing thefollowing:

 10 g Boric acid (Aldrich) 405 g Distilled water 110 g Isopropanol 475 gCabosperse PG008 (40 wt. % alumina; Cabot Corporation).

The coating dispersion contained 20 wt. % solids with a 95/5alumina/boric acid mass ratio. Two Mayer rods were used (one on eachsurface of the membrane) to control the wet layer thickness; and theseparator was then dried with a series of air knives, transportedthrough a vertical oven set at 80° C., and wound on a plastic core,prior to testing. Shrinkage of the coated separator in the machinedirection (MD) and the transverse direction (TD) was determined, asdescribed in Example 5. Table 18 shows the coating pickup, hightemperature thermal stability, and Gurley values for the coatedseparators. This example illustrates that an inorganic acid can beincorporated to provide excellent high temperature thermal stability ofthe coated separators. Additionally, this example illustrates theimportance of inorganic surface layer coating pickup on the thermalshrinkage properties of the coated separators.

TABLE 18 Coated separator characteristics Wt. Thickness pickup pickup200° C. shrinkage Gurley Composition Rod # (g/m²) (μm) MD % TD %(sec/100 ml) 95/5 alumina/boric acid #07/07 3.59 2.8 48.8 32.7 326 95/5alumina/boric acid #09/09 4.63 3.1 11.7 9.8 382 95/5 alumina/boric acid#12/12 6.14 4.6 3.5 1.6 357

Examples 19 and 20 relate to inorganic surface layer coatingformulations achieved by one or both of hydrogen bonding and chemicalreaction of the surface groups on the inorganic particles.

Example 19

A 12 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® GLP (Entek Membranes LLC,Oregon) was coated with an aqueous-based dispersion that contained thefollowing:

1000 g Distilled water  275 g Isopropanol 1172 g CAB-O-SPERSE PG 008 (40wt. % alumina; Cabot Corporation).

The coating dispersion contained 19.2 wt. % solids and only aluminaparticles. This coating dispersion is analogous to that of Example 5,with the exception that the resultant inorganic surface layer containsno organic hydrogen bonding component. Two #09 Mayer rods were used (oneon each surface of the membrane) to control the wet layer thickness; andthe separator was then dried with a series of air knives, transportedthrough a vertical oven set at 80° C., and wound on a plastic core,prior to testing. Shrinkage values of the coated separator in themachine direction (MD) and the transverse direction (TD) weredetermined, as described in Example 5. Table 19 shows the separatorcoating pickup, high temperature thermal stability, and Gurley valuesfor the coated separator,

TABLE 19 Coated separator characteristics Wt. Thickness Gurley Composi-pickup pickup 200° C. shrinkage (sec/100 tion Rod # (g/m²) (μm) MD % TD% ml) Alumina #09/09 5.44 3.5 4.2 2.1 343

Example 20

A 16 μm thick, microporous ultrahigh molecular weight polyethylene-basedseparator, Entek® KLP (Entek Membranes LLC, Oregon) was coated with anaqueous-based dispersion containing the following:

100 g Boehmite (AlO—OH) [5 wt. % in water; see J. Appl. Chem.Biotechnol. 1973, 23, 803-09 for preparation]  20 g Isopropanol 100 gCabosperse PG008 (40 wt. % alumina; Cabot Corporation).

The coating dispersion contained 20.5 wt. % solids with a 89/11alumina/boehmite mass ratio. The separator was dip-coated into a bathcontaining the aqueous-based dispersion. The coated polyolefin membranewas then dried in an oven set to 80° C. for 30 minutes prior to testing.Shrinkage of the coated separator in the machine direction (MD) and thetransverse direction (TD) was determined, as described in Example 5.Table 20 shows the coating pickup, high temperature thermal stability,and Gurley values for the coated separator.

TABLE 20 Coated separator characteristics Wt. Thickness Coating pickuppickup 200° C. shrinkage Gurley Composition (g/m²) (μm) MD % TD %(sec/100 ml) 89/11 alumina/ 19.8 15.6 5.3 5.3 387 boehmite

Applicants believe that the inorganic surface layers containing noorganic hydrogen bonding component bond to the separator as describedbelow. The porous particles in the inorganic surface layer arecharacterized by open chainlike morphology to form a virtual network atthe surface of the polyolefin membrane. The particles of the inorganicsurface layer are held together by particle-to-particle contacts thatinclude mechanical interlocking and hydrogen bonding. Preferred metaloxide particles include fumed alumina, silica, titanic, and zirconia.The inorganic surface layer is thought to be held to the separator bymechanical interlocking to its surface pores.

The following example demonstrates the effect of heat treatment on theadhesive strength of coated separators.

Example 21

Two different separators were used to study the effect of heat treatmenton the inorganic surface layer adhesive strength to the polyolefinmembrane. In the first case, the inorganic surface layer contained onlyalumina particles (see Example 19). In the second case, the inorganicsurface layer was prepared from an aqueous-based coating dispersionhaving a 95/5 alumina/boric acid mass ratio (see Example 18).

To study the effect of heat treatment on coating adhesion strength,three different conditions were employed:

-   -   1) a control condition in which no heat treatment was performed;    -   2) a heat treatment using calendar rolls (Innovative Machine        Corp.), with a gap set to 20 μm, a roll temperature of 125° C.,        and a roller speed of 1 ft/minute (30.5 cm/minute); and    -   3) en oven heat treatment at 125° C. for 4 hours in vacuum.

An inorganic surface layer adhesive strength test was performed, inwhich each coated separator was placed horizontally on a steel plate andmagnetic strips were placed on the edges of the separator to secure theseparator. A pressure sensitive tape (3M Scotch® Magic™ Tape 810, ¾ inch(1.9 cm) width), was applied to the coated separator. The free end ofthe tape was secured to a fixture dip, and the tape was peeled at 180°from the original tape orientation (i.e., 180° peel test configuration)at a speed of 0.1 inch/second (2.54 mm/second) and a distance of 4.5inches (11.4 cm). A force gauge (Chatillon, DFGS-R-10) with a 10±0.005lbs. (4 kg±2.7 g) load cell capacity was used to measure the forcerequired to remove the inorganic surface layer from the base polyolefinmembrane, and the maximum load was recorded. The test was repeated atleast three times for each sample. All testing was performed at roomtemperature. FIG. 10 is a bar graph showing the results of a peelstrength test of the two coated separators that underwent (1) no heattreatment, (2) calender roll heat treatment at 125° C., and (3) ovenheat treatment at 125° C. in vacuum.

Results showed that both coated separators had improved inorganicsurface layer adhesive strength after heat treatment. A comparisonbetween heat treatments revealed that, the longer the residence time,the better the adhesion. Coated separators containing boric acid showedmuch improved adhesive strength after heat treatment compared to coatedseparators containing only alumina particles. Only small differences inGurley values were observed before and after heat treatment. For thesample containing 95/5 mass ratio alumina/boric acid, the average Gurleyvalue before oven heat treatment was 324 s compared to 352 s after ovenheat treatment. This example illustrates that heat treatment can be usedto improve the adhesive strength of the coating with only a minimaldecrease in air permeability.

The following example demonstrates the effect of corona treatment onadhesive strength and wetting.

Example 22

A 16 μm thick, microporous ultrahigh molecular weightpolyethylene-containing separator, Entek® KLP (Entek Membranes LLC,Oregon) was corona treated with an Enercon TL Max™ web surface treater.The corona treatment settings were adjusted to a Watt density of 3.99Watts/ft²/miry, gap distance of 0.06 inch (1.5 mm), and a speed of 65ft/min (19.8 m/min). After corona treatment, the surface energyincreased from 35 Dynes to 52 Dynes, and the water contact angledecreased from 86° to 56°.

Entek® 16 μm KLP membranes with and without corona treatment were passedthrough three different aqueous-based coating dispersions: (1) a coatingdispersion containing 20 wt. % solids with only alumina particles(Cabosperse PG008), (2) a coating dispersion containing 20 wt. % solidswith a 95/5 alumina/boric acid mass ratio, and (3) a coating dispersioncontaining 20 wt. % solids with a 95/10 alumina/boric acid mass ratio.Compositions for each of the aqueous-based dispersions are described inTable 21.

TABLE 21 Coating Compositions Isopropyl PG008 Description of Boric acid(g) DI water Alcohol Dispersion coating (Aldrich) (g) (g) (g) Aluminacoating 0 390 110 500 95/5 Alumina/ 10 405 110 475 Boric Acid 90/10Alumina/ 20 420 110 450 Boric Acid

Each of the separators was dip-coated through a bath containing theaqueous-based dispersion, and the thickness of the wet layer wascontrolled on each side with a #9 Mayer rod. The separator was thendried with a series of air knives, transported through a vertical ovenset at 80° C., and wound on a plastic core, prior to testing. Thermalshrinkage of the coated separator in the machine direction (MD) and thetransverse direction (TD) was determined, as described in Example 5.

Table 22 shows the coating weight/thickness pickup, high temperaturethermal stability, and Gurley values for the coated separators prepared.A higher weight/thickness pickup was seen when coating onto coronatreated separators as compared to when coating onto untreatedseparators. Additionally, there was a clear improvement in wetting inthe separator upon corona treatment. For example, when attempting tocoat the aqueous-based dispersion containing a 90/10 alumina/boric acidmass ratio on an untreated separator, the aqueous-based dispersionbeaded up, thus resulting in a very uneven coating with poor quality. Incontrast, when applying this same aqueous-based dispersion to the coronatreated separator, the coating was applied very smoothly, and thequality of the coating was much improved.

TABLE 22 Coated separator characteristics Corona Basis Wt. Thickness200° C. Shrinkage Gurley Coating Composition Treatment? g/m² μm MD % TD% Sec/100 ml Alumina (PG008) No 13.9 19.9 26.2 17.3 352 Alumina (PG008)Yes 14.6 20.9 16.3 11.4 350 95/5 Alumina/Boric Acid No 14.2 20.6 17.79.8 332 95/5 Alumina/Boric Acid Yes 15.6 21.9 3.1 1.6 362 90/10Alumina/Boric Acid No Coating beaded 90/10 Alumina/Boric Acid Yes 14.522.2 3.1 2.6 358

The inorganic surface layer adhesive strength was determined using thepeel test method described in Example 21. Results are shown in Table 23,illustrating that the inorganic surface layer adhesive strength wassignificantly improved when the corona treatment was applied.Additionally, formulations with higher concentrations of boric acidresulted in more substantial improvements in the inorganic surface layeradhesive strength. This example illustrates the ability to improveadhesion of the inorganic surface layer and wetting of the coatingdispersion when corona treatment is applied to the base polyolefinmembrane.

TABLE 23 Effect of corona treatment on peel strength of coatedseparators Coating Peel Stength (lbs) Composition No Treatment (SD)Corona Treatment (SD) Alumina 0.025 0.003 0.043 0.008 95/5 Alumina/0.023 0.003 0.082 0.012 Boric Acid 90/10 Alumina/ — — 0.182 0.003 BoricAcid

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. For example,an inorganic surface layer may be applied as a coating on a portion(e.g., a patterned coating) of the surface or the entire surface of apolyolefin membrane. The scope of the invention should, therefore, bedetermined only by the following claims.

The invention claimed is:
 1. A freestanding polyolefin web, comprising:a microporous polyolefin membrane having a surface and a bulk structure,the polyolefin membrane characterized by a melting point; an aqueousdispersion-formed porous inorganic surface layer containing an organichydrogen bonding component, and inorganic particles which are at leastpartially coated with and uniformly dispersed within the organichydrogen bonding component, wherein the inorganic particles comprisefumed inorganic particles and other inorganic particles consisting ofcolloidal inorganic particles, boehmite, or combinations thereof,wherein the porous inorganic surface layer containing about 50 wt % toabout 99 wt % fumed inorganic particles, less than or equal to 3 wt % ofthe organic hydrogen bonding component, and the balance of which beingthe other inorganic particles, wherein the fumed inorganic particleshave a mean aggregate size of about 100 nm to about 300 nm and aspecific surface area of about 50 m²/g to about 225 m²/g, wherein theporous inorganic surface layer covers at least a portion of the surfaceof the polyolefin membrane, wherein the organic hydrogen bondingcomponent comprises a polymeric organic hydrogen bonding component, amolecule with multiple hydrogen bonding sites, or a combination thereof,and wherein the polyolefin web exhibits in-plane dimensional stabilityabove the melting point of the polyolefin membrane.
 2. The polyolefinweb of claim 1, in which the inorganic surface layer further includes aninorganic acid.
 3. The polyolefin web of claim 1, in which the moleculesare selected from a group including sucrose, maltitol, catechol,pentaerythritol, tannic acid, and dimethylol dihydroxyethylene urea. 4.The polyolefin web of claim 1, in which the other inorganic particlesinclude boehmite.
 5. The polyolefin web of claim 1, wherein themicroporous polyolefin membrane comprises a polyolefin matrix andcolloidal inorganic particles distributed therein.
 6. The polyolefin webof claim 1, in which the polyolefin web exhibits pore collapse in thepolyolefin membrane and less than 5% shrinkage in either of its in-planeaxes at 50° C. above the melting point of the polyolefin membrane. 7.The polyolefin web of claim 1, in which the fumed inorganic particlescomprise fumed silica, fumed alumina, or a combination thereof.
 8. Afreestanding polyolefin web, comprising: a microporous polyolefinmembrane having a surface and a bulk structure, the polyolefin membranecharacterized by a melting point; an aqueous dispersion-formed porousinorganic surface layer containing an organic hydrogen bondingcomponent, and inorganic particles which are at least partially coatedwith and uniformly dispersed within the organic hydrogen bondingcomponent, wherein the porous inorganic surface layer contains about 97wt % to about 99 wt % fumed inorganic particles and less than or equalto 3 wt % of the organic hydrogen bonding component, wherein the fumedinorganic particles have a mean aggregate size of about 100 nm to about300 nm and a specific surface area of about 50 m²/g to about 225 m²/g,wherein the porous inorganic surface layer covers at least a portion ofthe surface of the polyolefin membrane, wherein the organic hydrogenbonding component comprises a polymeric organic hydrogen bondingcomponent, a molecule with multiple hydrogen bonding sites, or acombination thereof, and in which the polyolefin web exhibits in-planedimensional stability above the melting point of the polyolefinmembrane.
 9. The polyolefin web of claim 8, in which the inorganicsurface layer further includes an inorganic acid.
 10. The polyolefin webof claim 8, in which the molecules are selected from a group includingsucrose, maltitol, catechol, pentaerythritol, tannic acid, anddimethylol dihydroxyethylene urea.
 11. The polyolefin web of claim 8, inwhich the microporous polyolefin membrane comprises colloidal inorganicparticles distributed throughout the bulk structure.
 12. The polyolefinweb of claim 8, in which the polyolefin web exhibits pore collapse inthe polyolefin membrane and less than 5% shrinkage in either of itsin-plane axes at 50° C. above the melting point of the polyolefinmembrane.
 13. The polyolefin web of claim 8, in which the fumedinorganic particles comprise fumed silica, fumed alumina, or acombination thereof.
 14. A freestanding polyolefin web, comprising: amicroporous polyolefin membrane having a surface and a bulk structure,wherein the microporous polyolefin membrane comprises a polyolefinmatrix characterized by a melting point; an aqueous dispersion-formedporous inorganic surface layer containing an organic hydrogen bondingcomponent, and inorganic particles which are at least partially coatedwith and uniformly dispersed within the organic hydrogen bondingcomponent, wherein the inorganic particles comprise fumed inorganicparticles and other inorganic particles consisting of colloidalinorganic particles, boehmite, or combinations thereof, wherein theporous inorganic surface layer containing about 50 wt % to about 99 wt %fumed inorganic particles, less than or equal to 3 wt % of the organichydrogen bonding component, and the balance of which being the otherinorganic particles, wherein the fumed inorganic particles have a meanaggregate size of about 100 nm to about 300 nm and a specific surfacearea of about 50 m²/g to about 225 m²/g, wherein the porous inorganicsurface layer covers at least a portion of the surface of the polyolefinmembrane, and wherein the polyolefin web exhibits in-plane dimensionalstability above the melting point of the polyolefin membrane.
 15. Thepolyolefin web of claim 14, in which the inorganic surface layer furtherincludes an inorganic acid.
 16. The polyolefin web of claim 14, in whichthe the organic hydrogen bonding component comprises a molecule withmultiple hydrogen bonding sites selected from a group including sucrose,maltitol, catechol, pentaerythritol, tannic acid, and dimethyloldihydroxyethylene urea.
 17. The polyolefin web of claim 14, in which theother inorganic particles include boehmite.
 18. The polyolefin web ofclaim 14, wherein the organic hydrogen bonding component comprises apolymeric organic hydrogen bonding component, a molecule with multiplehydrogen bonding sites, or a combination thereof.